Re: [PATCH] mm: implement and use vma_is_faulted(), silence KCSAN

[David Hildenbrand (Arm)]

On 9/9/26 18:14, Lorenzo Stoakes (ARM) wrote:
> Provide a function to abstract the common task of checking whether
> a VMA is faulted in or not.
> 
> A VMA or mmap lock must be held when calling this function. For an attached
> VMA the transitions between unfaulted/faulted state are:
> 
>        Transition        |              VMA/mmap Lock state
>  ------------------------|-----------------------------------------------
>    unfaulted to faulted  | write lock OR read lock + mm->page_table_lock
>    faulted to unfaulted  | write lock
> 
> So vma_is_faulted() never provides a false positive (the lock precludes
> it), but if only a read lock is held, a negative result must be re-checked
> with mm->page_table_lock held.
> 
> Detached VMAs cannot be concurrently manipulated as they are removed from
> the maple tree so require no guarantees.
> 
> Use data_race() to silence KCSAN about non-existent data races between
> concurrent vma->anon_vma read/write on optimistic fault tests.
> 
> Also while here, const-ify vma_is_attached(), vma_assert_stabilised() and
> dependants.
> 
> Finally, update the core VMA merge/split, rmap, mremap, KSM and fault
> preparation callers which test vma->anon_vma directly to use
> vma_is_faulted() instead.
> 
> Note that the lockless read in reusable_anon_vma() is doing more than
> checking whether the VMA is faulted - it is returning the anon_vma to be
> used on fault, so this check is not altered.
> 
> There is one odd one out - file_backed_vma_is_retractable() - which holds
> neither a VMA nor mmap lock and is stabilised by the file rmap lock only.
> 
> Therefore just add a comment to explain why the direct vma->anon_vma check
> is required.
> 
> Reported-by: Guilherme Giacomo Simoes <trintaeoitogc@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260829100034.423064-1-trintaeoitogc@xxxxxxxxx/
> Closes: https://lore.kernel.org/all/20260909115723.528501-1-trintaeoitogc@xxxxxxxxx/
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---


Is vma_is_faulted() really the right thing to use when wanting to say that we
(likely) faulted in an anon page?

I think that's highly confusing, considering just MAP_SHARED mappings where that
will never be true.

-- 
Cheers,

David